Radar systems are suited to contactless localization and distance determination of objects. Among a multiplicity of different radar systems, e.g. a frequency-modulated continuous wave radar system, also referred to in abbreviated form as FMCW radar, permits reliable distance determination between objects which are also e.g. spaced only a short distance apart from one another. Continuous wave (CW) radar radiation is typically used in such applications, rather than pulsed radar.
In frequency-modulated continuous wave radar systems, the frequency of the transmitted radar signals is subject to a periodic modulation, whereby transmitted radar signals that are reflected by an object exhibit, upon reentering the radar system, a shift in frequency compared to the transmitted radar signals, said frequency shift corresponding to a transit time difference. Said transit time difference represents twice the distance between the transmit and receive module and the reflecting object.
In order to localize or determine the distance of the reflecting object, received radar signals are typically subjected to spectral analysis in the receive module. For example, individual peaks in the power spectrum of the frequency shift shed direct light on the distance between the transmit and receive module and the reflecting object. Thus, for example, large frequency shifts correspond to large differences in transit time of the radar signals and consequently to great distances, whereas small frequency shifts correspond to small transit time differences and consequently to short distances.
The use of a radar system for determining the distance to a specific object is sometimes problematic, since multifarious external factors can have a substantial adverse effect on or interfere with the reception of reflected radar signals. Particularly in the presence of reflecting metal surfaces or bodies which absorb radar beams it is only possible to a limited degree to assign a received radar signal to a specific reflecting object. This problem can be at least mitigated by use of a transponder or a transponder device.
A transponder device is typically able to send a modulated radio response signal in response to an interrogation signal. If the receiver of the radar system is tuned to the modulation of the radio response signal, the radio response signals sent by a transponder device can be clearly separated from background and other interference signals with the aid of suitable filter methods and evaluated in order to determine the distance. Transponders can be embodied as passive or as active, i.e. power-amplifying, transponders.
The radio response signals modulated by a transponder device enable the distance between the transponder device and the receive module of the radar system to be determined. The use of a single transponder appears disadvantageous in particular in view of constantly changing ambient conditions. If, for example, the transponder and the receive module of the radar system are moving or absorbing or shadowing bodies are temporarily situated between the transponder and the receive module, this causes massive disruption to the reception of radio response signals as well as to the sending of interrogation signals to the transponder, with the result that determining a distance is no longer possible under certain conditions.
In order to localize or, as the case may be, determine the distance to a large, spatially extended object such as e.g. a machine, it is therefore beneficial to arrange a plurality of transponders around the machine so as to ensure an exchange of radio signals between the transmit and receive module and at least one transponder will be possible at any time.
Known from the prior art is a continuous wave radar system that has a spatially distributed arrangement of different transponders and can assign a radio response signal explicitly to one of the transponders. The radio response signal is subjected to a first modulation in the transponder so that the receive module can perform a distance determination between the transmit and receive module and the transponder. Furthermore, the radio response signal is subjected to a second modulation in the transponder in order to perform a data transmission between the at least one transponder and the transmit and receive module. In this way, for example, a transponder ID can be transmitted in order to identify the corresponding transponder with certainty.
In this case an interrogation signal which is transmitted by the transmit and receive module and in response to which the transponder sends the radio response signal, is a frequency-modulated continuous wave signal (FMCW signal). In other words the FMCW signal is used both for a data transmission and for measuring a distance. The interrogation signal is typically transmitted continuously in order to search for transponders, i.e. even when no transponders are present for communicating with the transmit and receive module. Since, however, the frequency of an FMCW signal is subject to a periodic modulation, it is very probable that adjacent transmit and receive modules which continuously transmit an FMCW signal will mutually affect one another, with the result that no reliable distance measurement can be performed. Furthermore, a transmit and receive module which constantly transmits an FMCW signal can interfere with other systems operating in the same frequency range, for example a WLAN system. Of course, these problems also occur when the FMCW signal is transmitted only temporarily. Moreover, an FMCW signal can also interfere with a data transmission in a continuous wave radar system from a transponder to a transmit and receive module, since the interrogation signal is typically very much stronger than the radio response signal.
A communication system which can be operated in two different modes is described in KONNO K ET AL: “60 GHz Millimeter-Wave Dual Mode Radar for IVHS” Topical Symposium On Millimeter Waves, 1997, Kanagawa, Japan, 7-8 Jul. 1997, pages 159-161, ISBN: 978-0-7803-3887-6. A base station transmits an unmodulated continuous wave signal in a first mode and a frequency-modulated continuous wave signal in a second mode, a mode selection signal of the base station determining the operating mode.
U.S. Pat. No. 6,868,073 B1 describes a distance measurement between two transponders. A first transponder sends an unmodulated signal to a second transponder in order to determine its identity. The second transponder sends its identity by means of a response signal to the first transponder, which checks the validity of the identity. If the validity is confirmed, the first transponder sends an acknowledgement to the second transponder, which after receiving said acknowledgement switches into a distance determination mode.
EP 1 903 412 A1 describes a method for enabling the operation of automation components of a technical system by means of a mobile control and monitoring device. The control and monitoring device receives a first identifier of a first transponder device. The control and monitoring device then sends a frequency-modulated carrier signal which is reflected by the first transponder device, thereby enabling the control and monitoring device to determine the distance to the transponder device.